1. Field of the Invention
The present invention relates to a frequency discrimination type torque tester for use in determining the quality of a bearing and more particularly to a frequency discrimination type torque tester having the function of judging a main cause of variation in torque which has been measured.
2. Description of the Prior Art
Heretofore, the measurement of torque has been conducted as one means for evaluating the quality of a bearing. According to this known means, while an outer race and an inner race of a bearing are rotated relative to each other, a rotational torque transmitted from one race to the other is detected as a load by means of a load cell, then the values measured in this way are expressed in terms of a chart wherein the measured values are arranged in time series, and the quality of the bearing is evaluated using maximum, minimum and mean values of the measured values on the chart.
The details of the above bearing quality evaluating means will now be described. FIG. 6 is an exploded view of a portion of the conventional evaluating means in which bearings as test samples are set to the same means. FIG. 7 is a sectional view showing a state in which the bearing is set and a rotational torque is measured by a load cell. As shown in FIG. 6, outer races of bearings 100 and 100 as test samples are fitted and held inside a cylindrical holding case 101 to constitute fulcrum bearings, while a threaded fitting cylinder 102 matching the inside diameter of the said fulcrum bearing is fitted in inner races of the bearings 100 and 100. A mounting screw 104 is inserted into a central bore 103 of the threaded fitting cylinder 102, and a threaded portion 105 thereof is brought into threaded engagement with a tapped hole 107 formed in the center of a fulcrum arm 106, whereby the fulcrum arm is mounted to the fulcrum bearing.
A collet 108 matching the outside diameter of the fulcrum bearing thus assembled is fitted on the fulcrum bearing with the fulcrum arm 106 attached thereto (see FIG. 7) and is connected to a rotary part of a motor 109. In this state the motor 109 is rotated and a load transmitted from the fulcrum arm 106 to a load cell 110 is measured, as shown in FIG. 7, which measured value is used as the torque of the inner race of the fulcrum bearing. As shown in a block diagram of FIG. 8, a torque tester for evaluating the quality of the bearings on the basis of the torque thus measured mainly comprises a motor controller 111 for controlling the rotation of the motor 109, an amplifier 112 for amplifying a torque signal which is outputted from the load cell 110, and a recorder 113 for recording the torque signal to evaluate the fulcrum bearing.
In the system thus constructed, if the fulcrum bearing is an ideal fulcrum bearing completely free of friction between the outer race and balls and also completely free of friction between the balls and the inner race, the torque of the inner race will be zero even if the outer race is rotated. Actually, however, there occurs such friction, so the load cell 110 detects the load and the torque tester outputs the detected torque value to the recorder 113 in accordance with the rotational speed of the outer race. FIG. 9 shows an example of outputs from the torque tester to the recorder in the prior art. FIG. 10 is a schematic diagram for explaining the phenomenon shown in FIG. 9. As shown in FIG. 10, time-series changes in torque value are classified into undulated changes and spiky changes according to, for example, deviation from roundness, rotational deflection, and finishing accuracy of the race surfaces of the bearing. Changes based on deviation from roundness and rotational deflection are gentle undulated changes observed with the lapse of time. On the other hand, changes based on finishing accuracy of the race surfaces of the bearing or a dent or dust which has got into the bearing are short spiky changes observed with the lapse of time. As a matter of course, it is preferable from the standpoint of quality that the degree of such changes be as small as possible.
According to the above conventional torque tester, maximum value, minimum value, maximum value minus minimum value, and mean value, are obtained on the torque chart recorded in the recorder 113 and the quality of the bearing as a test sample is evaluated on the basis of the values thus obtained. To be more specific, in the conventional method, as shown in FIG. 10, an upper limit value 1 and a lower limit value 1 are set for each data, and if the measured torque exceeds these limit values, the test sample concerned is regarded as being defective. According to this method, however, although a test sample containing such a component as spike 1 in FIG. 10 can be judged to be defective, a test sample containing a spiky component not exceeding the limit values such as spike 2 cannot clearly be judged to be defective from the torque chart in question despite it being defective.